Molded epoxy current transformer



Sept. 20, 1960 c. 1. YARRlcK Erm. 2,953,757

MOLDED EPOXY CURRENT TRANSFORMER Filed June 4, 195e A Trae/w75 UnitedStates Patent O MOLDED EPOXY CURRENT rrit/wsrfoRMER Charles J. Yarrick,Haddonfield, NJ., and John P. Green, Philadelphia, Pa., assignors toI-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Penn-Sylvania Filed June 4, 1956, Ser. No. 589,256

2 Claims. (Cl. 336--65) Our invention relates to a current transformersuitable for high voltage uses and the method of making such atransformer. More particularly, it relates to a current transformer, thesecondary Winding and magnetic core of which are completely insulated byencapsulation in an epoxy resin, and to the method of making such atransformer.

In the present method of constructing current transformers, it is thepractice to insulate between primary and secondary windings and toinsulate both windings from the core. Means for mounting the transformerare provided by metallic members attached to accessible parts of thecore. The terminals are brought out of the transformer by suitableinsulators. The magnetic core is available for grounding.

We have provided in the present invention a simplified method for makinga completely insulated self-supporting transformer having no metallicparts exposed. Through the use of casting resins, we have overcome therelatively expensive and complicated procedures of the prior art and areable to economically make a compact transformer having good mechanicaland electrical properties. The transformer of the present inventionrequires less space for mounting, has excellent resistance to moistureand chemically polluted atmospheres, and is of consistently good qualityand dimensional uniformity. These advantages are made possible byproviding a construction in which the transformer, except for theprimary winding, is completely sealed in cast epoxy resin.

We have also provided in the present invention cushioning means designedto avoid or greatly minimize any thermal stresses set up in thetransformer core during the casting operation which would otherwise tendto reduce its permeability.

Accordingly it is an object of our invention to provide a currenttransformer which has excellent insulating properties, resistance tohumid and chemical atmospheres, and is economical to manufacture.

Another object of our invention is to provide a current ,transformerhaving a secondary assembly which is a completely insulatedself-Supporting unit having no metallic parts exposed.

Still another object of our invention is to provide a novel andeconomical method for providing a current transformer with a cast ormolded epoxy insulation.

Another object is to provide cushioning means for avoiding stresses inthe magnetic core of a cast epoxy current transformer.

These and other objects of our invention will become apparent 'whentaken in connection with the drawings in which:

Figure l shows how the molded epoxy transformer of the present inventionwould appear when assembled.

Figure 2 shows the secondary and core of the transf former as it wouldappear just after being placed in the mold half.

Figure 3 is a Eside view of the present transformer parice 2 tially cutaway so that a portion of the secondary assembly is illustrated incross-section.

Figure 4 is a sectional view of a secondary winding and core sectiontaken along the line 4--4 of Figure 3 and looking in the direction ofthe arrows.

Essentially, the present invention relates to a completely insulatedself-supporting current transformer having no metallic parts exposed.The transformer is of the window or through type, comprising a singleturn primary conductor which passes through a secondary winding and coreassembly. The secondary in this instance is wound around a rectangulariron core, each corner of which is supported by a pre-cast epoxymounting boss. The core corners are cushioned by means of crumpledaluminum foil provided with a coating of silicone rubber. Layers ofglass mat are lashed to the winding as reinforcement at locations havingabrupt changes in resin thickness. The entire transformer isencapsulated in epoxy resin.

The secondary core legs are mounted in metallic supporting frames priorto the casting step to which are attached the pre-cast epoxy mountingbosses. Also, prior to the casting step the core corners are providedwith a cushioning of crumpled aluminum foil having thereon a coating ofsilicone rubber. The secondary assembly is then placed into a mold whichis then completely filled with liquied epoxy casting resin. The surfaceof the transformer after casting then consists of this cast resin alongwith the pre-cast mounting bosses. No metallic parts of the secondaryassembly are exposed and only the single turn primary bus bar and thesecondary wiring extends from the molding.

Referring now to the drawings in more detail, the transformer is of thewindow type and has a bus bar or single turn primary 10 passingtherethrough. The window 11 may be of any desired shape to accommodatethe bus bar. The surface of the secondary comprises cast epoxy resin 14and extending therefrom the four mounting bosses 1S, 16, 17 and 18. Theresin is preferably reinforced by glass mat 19.

The insulated wire terminals 20 and 21 also protrude from the secondary.Aside from these elements, all that is exposed to the atmosphere is thecast epoxy insulating resin. The line Z3 is merely excess epoxy resinwhich may be present on the surface at the point where the two halves ofthe mold were joined.

As can be more clearly seen from Figures 2 and 3, the secondarycomprises -a metallic frame 27 from which extend a pair of core sections30 and 31 around which the secondary winding 34 is wound. The coresection 30 is supported at either end by mounting bosses 15 and 16 whilethe core section 31 is supported by mounting bosses 17 and l18. Thebosses fit into both sections of the metallic frames by holes providedtherein. Thus, the secondary and core assembly has a generallyrectangular shape, having on opposite sides thereof a pair of parallelcore sections upon which the secondary winding is wound, the core legs54 and 55 being supported in metallic frames comprising a pair ofparallel sections carrying suitable mounting holes for the bosses.

In Figure 4 a detailed section of a secondary land core is shown. Thus,the core section 31 is partially encased in a thermosetting resin bobbin32. This resin bobbin, which may, for example, be made of Bakelite,serves as a support and as insulation from the core for the secondarywinding 34. The secondary contains a predetermined number of turns andmay comprise a plurality of layers of secondary wire.

Each leg 54 and S5 of the core carries an aluminum core clamp or frame27. These clamps support screws 39 and 40 which are threaded into theirrespective brass inserts 42 and 43. Thus the mounting bosses are securedto the core prior to the molding process during which the entire unit isencapsulated in the epoxy resin 14.

The secondary assembly having the cast epoxy mounting bosses securedthereto is placed in a mold 50, a half section of which is shown inFigure 2. Although not shown, it is to be understood that the mold issupplied with a rectangular core for casting the window for the primarywinding. The mounting bosses rest on a recessed portion of the molddesigned to accommodate the same and after the mold is filled withcasting epoxy resin, the mounting bosses remain exposed, and are anintegral part of the molded transformer.

Referring again to Figures 3 and 4, it is seen that the secondary coreis provided with cushioning means comprising pieces of crumpled aluminumfoil 52 which are coated with a silicone rubber 53. The foil ispositioned as a pad on the indicated end sections of the four corecorners.

Unvulcanized silicone rubber (for example Dow Cornings Silastic-S-2007)is applied to the outside surfaces of the foil and adjacent core areas.The rubber thereby seals air spaces in the foil. By this means, we avoidstresses in the magnetic core which would otherwise be set up during theepoxy casting operation or subsequent temperature changes. Such stresseswould decrease the permeability of the core and would have deleteriouseffects on the accuracy of the molded transformer.

These stresses are normally due to the fact that the steel in the corewill expand and contract at rates different from that of the resinenvelope as temperature changes take place in the molded transformer.

While it might be supposed that the problem could be solved by providinga resin formulation matching the thermal rate of the steel, this is notpracticable because .(1) thermal expansion rates are discontinuousfunctions for all common materials, particularly for epoxy resins, and(2) to reduce the thermal coefficient of the epoxy would require theaddition of fillers which would raise the viscosity of the uncured resinto unworkable limits.

The present provision utilizing aluminum foil and silicone rubber whichis vulcanized after the cushioning is in place avoids these problems andat the same time Virtually eliminates the setting up of stresses in thecore during the casting operation, and subsequent use of thetransformer.

In applying the cushioning, it is also desirable to provide the othersurfaces of the core with a raw rubber coating of about one-thirtysecond of an inch and then vulcanizing the rubber at about 330 F. forone hour. The winding is then molded in the manner hereinafterdescribed.

The epoxy resins herein referred to are of the type `Comprising theresinous produce of reaction between an epihalohydrin such asepichlorohydrin and a polyhydroxyphenol such Ias bis-phenol-A. Resins ofthis type are referred to in United States Patents Nos. 2,324,483,2,444,333 and 2,458,796. These resins have been found to possessexcellent mechanical resistance, resistance to Water and alkali andoutstanding electrical insulation properties. Depending upon the:desired physical properties, various llers such as slate, quartz, intand clays may be incorporated with the epoxy resins.

An example of a formulation of a casting resin which has been found tobe particularly suitable is as follows:

29.2 percent by weight unmodified epoxy resin of the type suitable foracid catalysis. See for example, United States Patent No. 2,712,535.This resin goes under such trade names as Araldite 6060 and Epon 1001.

6.0 percent by weight of plasticizer such as a long-chain carboxylicacid of the type described in United States Patent No. 2,712,535 foradding flexibility and resilience to the epoxy resin. 7.80 percent byweight of phthalic anhydride as hardening or cross-linking agent.

57.0 percent by Weight electrical grade flint (98 percent through 200mesh screen, 99.8 percent pure silicon dioxide).

A similar formulation is used for the mounting bosses, except that theyare made to have a higher heat distortion rating and the plasticizer isomitted so that the formulation is as follows:

34 percent by weight unmodified epoxy resin. 10 percent by weightphthalic anhydride. 66 percent by weight flint.

The epoxy resin is melted and heated to 300 F. Then the plasticizer ismelted and heated to 300 F. and added to the epoxy. Predried flint, asabove indicated, is added to the mix, after which phthalic anhydridehardener is added. The whole is then thoroughly mixed for five minutes,the temperature being maintained at 300 F.

The mold 50 may be of any metal such as aluminum or brass which hasdimensional stability at the molding temperature of about 300 F. Theinside surface 51 of lthe mold should be polished. Moreover, it isgenerally desirable to provide the mold with a silicone release agent.Such agents are well known in the art. One example would be the siliconegoing under the name of DC 7 made by the Dow Corning Corporation. As asubstitute for the silicone release agents, standard hard and polishedsilver plating may also be used.

Prior to the molding operation, the mold is coated with the releaseagent. The assembled secondary after being provided with the cushioningmeans hereinabove referred to is dried for two hours at 320 F., in aforced circulation oven, The assembled mold is then placed in a heatedvacuum tank which is then evacuated to 29' to 29.5 inches of mercury andbetween 300 F. and 320 F. The casting resin mix is poured slowly intothe mold at 300 F. and the entire assembly is cured at atmosphericpressure for about sixteen hours at between 200 F. `and 260 F. Afterrelease from the mold, the cast unit is further cured for four hours atbetween 250 F. and 260 F. As the unit is cooled the contracting resincompresses the rubber which being resilient, will transmit the pressurefrom all core surfaces to the aluminum cushion thereby compressing theair contained therein. In any subsequent heating of the transformer theair expands und the motion is reversed. After the unit is completed, thewindow for the primary may be painted with a carbonaceous conductivepaint of a type well known in the art.

Although we have described preferred embodiments of our invention, manymodifications and variations will now be apparent to those skilled inthe art, and we prefer therefore, to be limited not by the specificdisclosure herein, but only the appended claims.

We claim:

l. A completely insulated, self-supporting current transformer having nometallic parts exposed comprising a core structure carrying a secondarywinding; a first and second portion of said ycore structure having afirst and second frame member secured thereto; said irst and secondframe members having a respective extending boss of insulating material;said core structure, said secondary winding and a portion of said bossesbeing encapsulated in an epoxy resin; said encapsulated core structurehaving an opening therein for receiving a single turn primary conductor;said bosses being at least partially exposed for mounting said currenttransformer.

2. The device substantially as claimed in claim l wherein said core hasa square configuration; the corners of said square configuration beingcushioned by a cushion of crumpled aluminum foil having a coating ofsilicone rubber to absorb stress during cooling of said encapsulatingepoxy resin.

(References on following page) 5 6 References Cited in the le of thispatent 2,731,607 Gould et al. 1..-- an. 17, 1956 2,756,399 August et alJuly 24, 1956 2175 934 UTEZ) STATES PATENTSO lo 1939 2,785,383 FosterMar. 12, 1957 en ct. 2,382,199 Brink Aug. 14, 1945 5 OTHER REFERENCES2,414,525 Hill et a1 Jan. 21, 1947 Ethoxylines, by Prieswerk, ElectricalManufacturing,

2,632,211 Trigg Mar, 24, 1953 July 1949, P1178 through 81

1. A COMPLETELY INSULATED, SELF-SUPPORTING CURRENT TRANSFORMER HAVING NOMETALLIC PARTS EXPOSED COMPRISING A CORE STRUCTURE CARRYING A SECONDARYWINDING, A FIRST AND SECOND PORTION OF SAID CORE STRUCTURE HAVING AFIRST AND SECOND FRAME MEMBER SECURED THERETO, SAID FIRST AND SECONDFRAME MEMBERS HAVING A RESPECTIVE EXTENDING BOSS OF INSULATING MATERIAL,SAID CORE STRUCTURE, SAID SECONDARY WINDING AND A PORTION OF SAID BOSSESBEING ENCAPSULATED IN AN EPOXY RESIN, SAID ENCAPSULATED CORE STRUCTUREHAVING AN OPENING THEREIN FOR RECEIVING A SINGLE TURN PRIMARY CONDUCTOR,SAID BOSSES BEING AT LEAST PARTIALLY EXPOSED FOR MOUNTING SAID CURRENTTRANSFORMER.